Conventionally, plants have been deeply involved with human as foods, ornaments, industrial materials such as paper and chemicals, and fuels. Further, recently, plants have been spotlighted as biomass energy that will substitute fossil fuel.
Although plants have been used in such various fields, their mechanisms such as germination, growth, and flowering have not yet been clarified in many regards. Consequently, cultivation of plants has been mainly based on experiences and intuition, and harvest of the plants has been greatly influenced by natural conditions such as weather. Therefore, clarification of plants' mechanisms such as germination, growth, and flowering and regulating and controlling the mechanisms are very important not only for increasing yields of ornamental plants and food plants such as cereals and vegetables, but also for growing woods in forests and biomass energy.
In order to regulate growth of plants, there have been made attempts such as regulation of flowering by artificial environments such as a conservatory, and promotion of growth by use of chemicals such as ethylene. However, most of these conventional attempts are regulations of growth of plants based on experiences and intuition, and are not based on data that allows scientific evaluation of growth of plants.
The inventors of the present invention have researched on the plant's mechanisms of germination, growth, and flowering. Consequently, the inventors have shown that a redox status adjusting substance such as reactive oxygen and glutathione is essential as a factor for controlling growth of plants (see Patent Literatures 1 and 2).
Glutathione (GSH) is a tripeptide that is synthesized in such a manner that γ-glutamylcysteine is synthesized from cysteine and glutamic acid by γ-glutamylcysteine synthetase and glycine is then added to γ-glutamylcysteine by GSH synthetase. Glutathione is a main intracellular antioxidant, and has a function of detoxifying a foreign matter in the cell.
It has been reported that a transformed plant to which a γ-glutamylcysteine synthetase gene of Escherichia coli is introduced is efficiently increased in glutathione content (see Non Patent Literatures 1 to 3). However, it has been also reported that such a transformed plant may become intolerant to light (see Non Patent Literature 3).
On the other hand, it has been reported that in a case where γ-glutamylcysteine synthetase gene of the plant itself is overexpressed, i.e., in a case of a transformed plant in which γ-glutamylcysteine synthetase gene of Arabidopsis thaliana is introduced into Arabidopsis thaliana and then overexpressed, the expression levels of γ-glutamylcysteine synthetase mRNA and translated product thereof (protein) are greatly increased, whereas the glutathione content is not so much increased (see Non Patent Literature 4). Therefore, it has been recognized that use of a plant-derived γ-glutamylcysteine synthetase gene is not a good method for increasing the glutathione content in the plant.